During the drilling of a bore hole in earth formations, it is often necessary to change the direction of the bore hole. Numerous tools exist for adjusting the direction of drilling. Adjustable bent subs, such as those disclosed by Wenzel (U.S. Pat. No. 4,745,982) and Wilson et al. (U.S. Pat. No. 5,029,654), are sometimes used to change the drilling direction. These subassemblies, however, do not permit adjustment of the bend angle between the axis of the drill string above the tool and the axis of the drill string below the tool once the subassembly is inserted in the bore hole. As a result, the drill string must be removed, or tripped, both for attachment of the subassembly and for removal of the subassembly once the direction of drilling has been changed.
Some tools permit limited changes in the deflection angle of the bore hole without tripping the drill string. In the tools disclosed by Kamp (U.S. Pat. No. 4,492,276), Geczy et al. (U.S. Pat. No. 4,667,751), Steiginga (U.S. Pat. No. 4,880,066), Delucia (U.S. Pat. No. 4,932,482) and Delucia (U.S. Pat. No. 4,962,818), the bend angle is preset either in the downhole motor housing or through the use of a bent sub. Directional drilling is achieved by rotating only the drill bit, and straight line drilling is achieved by rotating the drill bit and the down-hole motor and/or drill string. The drill string must be held stationary to hold a given directional angle, which is often difficult to achieve in practice. In addition, straight bore drilling through rotation of the drill string produces a bore hole diameter larger than needed or generated by other common drilling techniques. Furthermore, because it is fixed, the bend angle cannot be changed without tripping the drill string.
Other directional drilling tools are not limited to a preselected bend angle in the drill string. A change in the direction of drilling is achieved downhole, through the use of pistons, rollers or the like which are actuated once the drill string is in position, to either incline the drilling axis within the tool or incline the tool in the bore hole. Some of these tools, as disclosed by Page et al. (U.S. Pat. No. 2,891,769), cannot drill a straight bore hole, or, as disclosed by Evans (U.S. Pat. No. 4,291,773), cannot vary the predetermined bend angle while in the bore hole. Others, such as disclosed by Claycomb (U.S. Pat. No. 3,595,326), can vary only between a straight line and a preselected bend angle. Still others, as disclosed by Takaoka et al. (U.S. Pat. No. 4,046,204) and Edmond et al. (U.S. Pat. No. 4,281,723), can vary either the angle of bend from the longitudinal axis of the tool or the direction of bend about the longitudinal axis of the tool, but require separate control lines from the surface to the tool. All of these tools require, to varying degrees, complicated mechanical and pressurized fluid carrying parts.
Other subassemblies use the mud fluid pressure to effectuate changes downhole in the bend angle of the subassembly. The deviation tools of Schoeffler (U.S. Pat. No. 4,655,299) and Schoeffler (U.S. Pat. No. 4,895,214) each have an output shaft which can be caused to be alternately straight or bent by cycling the drilling fluid flow rate at the earth surface so as to cause a high or low flow resistance in the tool. The flow resistance can be detected at the earth surface for determining the configuration of the tool. However, each of these tools includes fragile spherical gimbals to permit pivoting of the output shaft.
Still other directional drilling tools permit downhole adjustment to a bend angle in the tool through the use of two cylindrical members which can be rotated with respect to one another to vary the bend angle from zero to an inherent maximum angle. Two such tools are disclosed in Wawrzynowski et al. (U.S. Pat. No. 4,884,643) and Smet (U.S. Pat. No. 5,002,138).
The tool in Wawrzynowski uses a subassembly coupled to the drill string comprising two cylinders each having a first straight portion which are concentrically coupled to each other and a second portion bent at the same oblique angle to the respective first portion and coupled to either the upper or lower portion of the drill string. Axial pumping of the upper drill string at a predetermined force causes incremental changes in the bend angle over a range from zero degrees to twice the oblique angle by rotating the cylinders with respect to each other at set angular amounts. An internal coil spring is provided for retaining the cylinders in the desired angular relationship. This device can be impractical and unworkable because the downward longitudinal forces on a drill string during operation can vary considerably thereby making it difficult to maintain at all times the necessary threshold force on the drill bit for retaining the desired bend angle. Furthermore, it has proven difficult to build a coil spring within the space available which can resist the normal and usual drill string downward forces on the drill bit without permitting transient compressions of the spring which can undesirably cause the bend angle to change.
In addition, the Wawrzynowski tool utilizes a complex rotator with various moving parts to accomplish the rotation. The tool does not include a device for signaling to the surface the position of the two cylinders with respect to each other, nor does the tool cause the longitudinal axis of the drill string above the tool to intersect at all times with the longitudinal axis of the drill string below the tool at the midpoint of the longitudinal centerline of the tool. This failure to so intersect can cause the drill bit to carve out an undesirable three-dimensional curved path rather than a simple two-dimensional path in the plane of the upper and lower drill strings.
Smet discloses a steerable drilling mole having a cylinder which has an oblique face and is rotatably driven about the central axis of the device. A ball is provided with an axially centered bore therethrough and an oblique face inclined at the same angle as the oblique face of the cylinder. The oblique face of the ball abuts the oblique face of the cylinder. Rotation of the cylinder in the housing causes the ball to rotate about an axis perpendicular to the central axis thereby adjusting the bend angle over a range from zero degrees to twice the angle of the oblique faces on the cylinder and ball. The driving motor, complex ball and socket joint and other fragile apparatus in the tool give rise to several sources of failure. In addition, the mole requires a separate high pressure line from the surface to operate the driving motor and an additional line to relay to the surface the position of the rotatable cylinder within the mole.
From the prior art, it is a clear that one of the goals in directional drilling is maximizing the rate of penetration and minimizing the costs of operation. Directional drilling tools which require tripping of the drill string to change the bend angle or to achieve directional drilling are not cost effective in most cases because of the accompanying delays in operation. Bend tools in the prior art which are adjustable down hole provide the opportunity for increased penetration rates, but are often accompanied by undesirable consequences. Many of these bend tools require dedicated control or other lines down hole which increase the costs of drilling. Most of these tools contain complicated and fragile mechanisms which are prone to failure and cause delays in the operation when not functioning correctly. The majority of these tools do not permit relatively unrestricted adjustment of the bend angle within a given range. During the operation of some of these tools, the longitudinal axes of the upper and lower drill strings are offset causing undesirable deviations in the bore hole. Furthermore, most of these tools have not considered the significant hydraulic forces within the tools which tend to separate the housings of the tool.
Variable relief valves have also been heretofore provided. Spring-loaded pressure relief valves, for instances, are commonly used to relieve excessive pressure from high pressure pumps and fluid lines. However, the common spring-loaded variable relief valve cannot tolerate high volume flow after it opens a short distance. Pressures above the relief pressure will maintain a relief valve in an open position, but the passage between the valve head and the valve seat is so narrow that serious erosion of the valve seat and head surfaces occurs very quickly.
As can be seen from the above discussion, there is a need for a bend tool adjustable downhole which requires no dedicated control or other lines downhole to adjust or measure the bend angle, is of a relatively simple and sturdy design with no fragile mechanical joints, can indicate at the well head the bend angle of the tool, and permits the longitudinal axis of the drill string above the tool to intersect on the longitudinal axis of the tool with the longitudinal axis of the drill string below the tool.